This invention relates to a heated galvanic-type solid electrolyte oxygen sensor, and more particularly to an improved and readily assemblable construction of such a sensor in which the heater is electrically isolated from sensor terminals.
Solid electrolyte galvanic oxygen sensors essentially include an oxygen-ion-conductive ceramic body with porous electrodes on opposite faces of the body. One electrode is exposed to a reference source of oxygen. The other electrode is exposed to a source whose oxygen content is to be measured. A difference in oxygen partial pressure at the electrodes results in a corresponding electrode potential difference, providing a sensor output voltage.
The output voltage of such sensors can be used to measure oxygen or unburned combustibles in combustion gases produced by an internal combustion engine. This voltage can be used in monitoring and controlling the combustion process, as disclosed in U.S. Pat. No. 3,616,274 Eddy, U.S. Pat. No. 3,844,920 Burgett et al and U.S. Ser. No. 787,900 Howarth, filed Apr. 13, 1977, now U.S. Pat. No. 4,129,099.
The solid electrolyte of such a sensor must be heated to an elevated temperature to obtain an appreciable output voltage. Also, sensor output voltage varies directly with changes in temperature, especially at lower operating temperatures. Combustion gases can be used to heat the sensor to operating temperatures but such gases vary widely in temperature, particularly when from an internal combustion engine. The aforementioned U.S. Pat. No. 3,616,724 Eddy discloses sensor temperature compensating means that includes a surrounding resistance heater. U.S. Pat. No. 3,815,560 Wahl et al discloses a surrounding resistive heater to maintain an electrolyte tube at high temperatures where its output voltage is least affected by temperature change. The aforementioned U.S. Ser. No. 787,900 Howarth now U.S. Pat. No. 4,129,099 discloses doping the solid electrolyte with iron oxide for temperature compensation. It additionally discloses disposing a resistance heater inside a solid electrolyte tube for maintaining the sensor at higher operating temperatures and for supplemental heating on start up.
For automotive applications, the heated sensor should be particularly rugged and reliable. In addition, for higher reliability and lower cost, the heated sensor should be simple and readily manufacturable. U.S. patent application Ser. No. 892,644 entitled "Heated Solid Electrolyte Oxygen Sensor", concurrently filed herewith in the name of Michael P. Murphy, a co-inventor herein, discloses a new way to incorporate a heater in the oxygen sensor, particularly an automotive oxygen sensor. His invention involves forming a subassembly of the heater and the sensor reference electrode terminal. In the subassembly, the heater is prealigned so that when the reference electrode terminal is assembled with its solid electrolyte, the heater is also inherently aligned with the solid electrolyte. In summary, Murphy proposes adding a heater to a solid electrolyte oxygen sensor as a subassembly with a reference electrode terminal for the solid electrolyte. The heater-electrode terminal subassembly is particularly useful in an oxygen sensor such as disclosed in U.S. Pat. No. 3,844,920 Burgett et al.
One specific construction for insulatingly supporting and aligning a heater with a reference electrode terminal is shown in FIGS. 5-8 of the aforementioned U.S. patent application Ser. No. 892,644, but not specifically claimed therein. This specific construction is not suggested by other embodiments described in U.S. patent application Ser. No. 892,644 or the invention claimed therein.
In this specific construction, the heater is electrically isolated from the electrode terminal. Moreover, the heater is not only supported in aligned disposition on the electrode terminal but precise alignment is assured, even though the components are readily assemblable. This construction includes use of a vitrified bond to secure the subassembly components together. A vitrified bond has reportedly been previously used in making electrically insulated coaxial transformer thru-the-wall connectors. We have found that a vitrified bond can provide a simple, rugged, reliable and readily manufacturable subassembly for a heated oxygen sensor. In addition, the heater in the resultant sensor is coaxial. All terminals are coaxial. Further, the heater is electrically isolated from the sensor electrodes. Hence, the heater can be controlled independently from sensor electrodes. Such a sensor is an improvement on the invention claimed in the aforementioned U.S. patent application Ser. No. 892,644.